Mirror via conductivity for DMD pixel

ABSTRACT

A method includes forming a first aluminum silicon layer on a metal layer and forming a titanium nitride layer (or other titanium-based layer) on a surface of the aluminum-silicon layer opposite the metal layer. The method further includes etching the titanium nitride layer to create a titanium nitride pad and forming a torsion hinge in the metal layer. The titanium nitride pad is on the torsion hinge. The method also includes depositing a sacrificial layer over the torsion hinge and titanium nitride pad, forming a via in the sacrificial layer from a surface of the sacrificial layer opposite the torsion hinge to the titanium nitride pad, depositing a metal mirror layer on a surface of the sacrificial layer opposite the torsion hinge and into the via, and removing the sacrificial layer.

BACKGROUND

Light processing systems often involve directing light towards a displaysuch that an image is produced. One way of effecting such an image isthrough the use of digital micromirror devices (DMD). In general, lightis shined on a DMD having an array of numerous micromirrors. Eachmicromirror is selectively controlled to reflect the light towards aparticular portion of a display, such as a pixel. The angle of amicromirror can be changed to switch a pixel to an “on” or “off” state.The micromirrors can maintain their “on” or “off” state for controlleddisplay times.

SUMMARY

In one embodiment, a method includes forming a first aluminum siliconlayer on a metal layer and forming a titanium nitride layer on a surfaceof the aluminum-silicon layer opposite the metal layer. The methodfurther includes etching the titanium nitride layer to create a titaniumnitride pad and forming a torsion hinge in the metal layer. The titaniumnitride pad is on the torsion hinge. The method also includes depositinga sacrificial layer over the torsion hinge and titanium nitride pad,forming a via in the sacrificial layer from a surface of the sacrificiallayer opposite the torsion hinge to the titanium nitride pad, depositinga metal mirror layer on a surface of the sacrificial layer opposite thetorsion hinge and into the via, and removing the sacrificial layer.

In yet another embodiment, a method includes forming a titanium nitridelayer on a metal layer, forming a first aluminum silicon layer on asurface of the titanium nitride layer opposite the metal layer, etchingthe titanium nitride layer to create a titanium nitride pad, and forminga torsion hinge in the metal layer. The titanium nitride pad is on thetorsion hinge. The method also includes depositing a sacrificial layerover the torsion hinge and titanium nitride pad, forming a via in thesacrificial layer from a surface of the sacrificial layer opposite thetorsion hinge to the titanium nitride pad, depositing a metal mirrorlayer on a surface of the sacrificial layer opposite the torsion hingeand into the via; and removing the sacrificial layer.

An apparatus also is disclosed that includes a semiconductor substrate,a torsion hinge formed on the semiconductor substrate, and a titaniumnitride pad formed on the torsion hinge. The apparatus also includes amirror layer including a via formed on the titanium nitride pad androtatable by the torsion hinge. The apparatus may comprise a DMD.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 illustrates a digital micromirror device (DMD) in accordance withthe disclosed embodiments;

FIG. 2 illustrates an initial material stack to form the DMD inaccordance with the disclosed embodiments;

FIG. 3 illustrates an intermediate step in which a bottom reflectivecoating (BARC) and an oxide layer have been removed in accordance withthe disclosed embodiments;

FIG. 4 illustrates a further intermediate step in which an aluminumsilicon layer has been removed in accordance with the disclosedembodiments;

FIG. 5 illustrates a further intermediate step in which a pillar patternis placed on the titanium nitride layer in accordance with the disclosedembodiments;

FIG. 6 illustrates yet a further intermediate step in which the titaniumnitride layer has been etched to form a titanium nitride pad inaccordance with the disclosed embodiments;

FIG. 7 shows an upper surface of a metal layer containing the titaniumnitride pad in accordance with the disclosed embodiments;

FIG. 8 illustrates a further step in which the metal layer has beenetched to form a hinge in accordance with the disclosed embodiments;

FIG. 9 shows a portion of the surface of the metal layer with the hingeformed thereon and the titanium nitride pad formed on the hinge inaccordance with the disclosed embodiments;

FIG. 10 shows an alternative embodiment of a material stack for formingthe DMD in accordance with the disclosed embodiments;

FIG. 11 shows an example of a via formed in a sacrificial layer and amirror metal layer formed on the sacrificial layer and into the via inaccordance with the disclosed embodiments;

FIG. 12 shows an alternative of the structure of FIG. 11;

FIG. 13 shows an embodiment in which the titanium nitride pad is formedto self-align to the hinge in accordance with the disclosed embodiments;

FIG. 14 shows the example of FIG. 11 following the removal of thesacrificial layers;

FIG. 15 shows the example of FIG. 12 following the removal of thesacrificial layers; and

FIG. 16 shows the example of FIG. 13 following the removal of thesacrificial layers.

DETAILED DESCRIPTION

A digital micromirror device (DMD) includes an array of mirrors (alsotermed micromirrors herein) with each mirror mechanically andelectrically coupled to a corresponding torsion hinge by way of aconductive via. Each mirror can be made to rotate to one position oranother (e.g., +/−1-12 degrees) through application of suitable voltagesto electrodes. Once the mirror has been rotated to one position (e.g.,+12 degrees), a sufficiently large voltage (the recapture margin) isrequired to rotate the mirror to the opposite position (e.g., −12degrees).

The disclosed embodiments of a DMD reduces the recapture voltage thatwould otherwise be needed to move the mirror. In an embodiment, the viathat mechanically supports the mirror over the hinge and provideselectrical conductivity to the mirror is formed on the hinge with atitanium nitride (TiN) pad and aluminum silicon layer creating an ohmicpath between the mirror metal and and the hinge metal. Alternativeembodiments include titanium-tungsten (TiW) or titanium-aluminum nitride(TiAlN) instead of TiN. The titanium-based pad (e.g., TiN pad) preventsnative oxide growth on the hinge metal and reduces the resistance of theinterface between the mirror metal and the hinge metal in the viainterconnect thereby permitting the mirror to be rotated with a smallervoltage.

FIG. 1 illustrates a single DMD pixel element 200. An array of suchpixel elements may be formed on a common semiconductor die. The DMDpixel element 200 of FIG. 1 may include a hinge portion, an addressportion, and a mirror portion. The hinge portion includes a hinge 216(which may be a torsion hinge), supported on each side by hinge posts.Six bias vias 208 support spring tips 226 (two of which are shown inFIG. 1 and two more are present but hidden in this view) and hinge 216above the lower layer 230. The bias vias 208 may also operate to relay abias voltage to hinge 216. A micromirror 204 is supported above thehinge 216 by a mirror via 202. A titanium nitride pad 314 a issandwiched between the via 202 and the hinge 216 to thereby reduce theresistance of the mirror via 202. In addition to providing support forthe micromirror 204, the mirror via 202 may conductively transfer thebias voltage to the micromirror 204. The bias voltage may then beconductively transferred to the spring tips 226 and hinge 216 throughthe six bias vias 208. The bias voltage may be then further transferredfrom the hinge 216 to the micromirror 204 through the mirror via 202.

The address portion of the DMD pixel element 200 includes two addresspads 212 a, 212 b (collectively address pads 212) that each connect toraised address electrodes 214 a, 214 b, respectively. As illustrated inFIG. 1, address vias 213 support the raised address electrodes 214 a,214 b above each address pad 212 a, 212 b. In addition to supporting theraised address electrodes 214 a, 214 b, the address vias 213 relay acontrol or address voltage from the address pads 212 a, 212 b to theraised address electrodes 214 a, 214 b. The address pads 212 a, 212 bmay be in communication with a control circuitry, such as a staticrandom access memory (SRAM) cell or the like, which selectively appliesa control or address voltage to one of the two address pads 212 a, 212 bto create an electrostatic force between the micromirror 204 and theraised address electrodes 214 a, 214 b. A similar electrostatic forcemay be created between the micromirror 204 and the address pads 212 a,212 b.

The range of motion of the micromirror 204 may be limited by spring tips226. During operation of DMD pixel element 200, spring tips 226 providea landing point for micromirror 204. For example, when micromirror 204is tilted in the direction of the raised address electrode 214 a andaddress pad 212 a, the spring tips 226 positioned proximate theseaddress elements will operate as a landing point for micromirror 204.Conversely, when micromirror 204 is tilted in the direction of theraised address electrode 214 b and address pad 212 b, the spring tips226 on the opposite side (and hidden in the view of FIG. 1) positionedproximate these address elements may operate as a landing point formicromirror 204. Thus, micromirror 204 may be tilted in the positive ornegative direction until the micromirror 204 contacts one or more springtips 226.

As noted above, a titanium nitride pad is included between the mirrorvia 202 and the hinge 216. In some embodiments, each DMD pixel elementincluding the titanium nitride pad is formed by way of one or moresemiconductor process operations, examples of which are provided below.

FIGS. 2-13 illustrate the DMD pixel element at various stages offormation. The processing operations include fabricating a titaniumnitride pad at the interface between the mirror via and the hinge. FIG.2 illustrates a stack of materials. In this example, the stack includesa titanium nitride layer 302, a silicon nitride layer 304, an aluminumoxide (Al₂O₃) layer 306, a sacrificial layer 308, a metal layer 310, analuminum silicon layer 312, a titanium nitride layer 314 (whichultimately is used to implement the titanium nitride pad), anotheraluminum silicon layer 316, an oxide layer 318, and a bottomantireflective coating (BARC) 320. The various layers can be depositedone on top of the other using any of a variety process operations. Thesacrificial layer 308 may comprise a photoresist. The metal layer 310,aluminum silicon layer 312, titanium nitride layer 314, and aluminumsilicon layer 316 may be sputtered in the same tool in three differentprocess chambers under vacuum so that no native oxide is grown betweenthe layers. The deposition of layer 316 in-situ is optional and can beperformed in another tool after an air break. Native oxide is not aproblem for the interface between the titanium nitride layer 314 and theupper aluminum silicon layer 316.

The metal layer 310 eventually is etched to form the hinge and thetitanium nitride layer 314 is processed to form a titanium nitride padon the hinge. Aluminum silicon layers 312 and 316 are provided on eitherside of the titanium nitride layer 314 for different reasons. The bottomaluminum silicon layer 312 protects the metal layer 310 during theetching process of the titanium nitride layer 314 to form the titaniumnitride pad. The lower aluminum silicon layer 312 functions as an etchstop layer to protect the metal layer 310. The subsequent wet chemicaletch with developer exposes the metal layer 310 with nearly no change inthickness, thereby not interfering with the characteristics of the hinge216. The upper aluminum silicon layer 316 functions to provide an etchstop layer for the oxide etch process. This prevents the formation of anundesirable hard to etch titanium fluoride layer. After a wet chemicaletch with developer, the next pattern and etching steps produce a smoothsurface of the titanium nitride pad and hinge 216.

The thicknesses of the aluminum silicon layers 312 and 316 and thetitanium nitride layer 314 can vary from embodiment to embodiment. Insome embodiments, the thickness of the lower aluminum silicon layer 312is approximately 200 {acute over (Å)}, the thickness of the upperaluminum silicon layer 316 is approximately 100 {acute over (Å)}, andthe thickness of the titanium nitride layer 314 is approximately 50{acute over (Å)}. The metal layer 310 may be approximately 350 {acuteover (Å)} thick in some embodiments.

The BARC 320 is dry etched from the flat surfaces of the wafer but BARCis left in the via(s) since the BARC coat process causes BARC to bethicker in the via(s). The oxide layer 318 is dry etched from the flatsurfaces of the wafer but the BARC that is left in the via(s) protectsthe oxide and the other layers in the via, the upper aluminum siliconlayer 316, titanium nitride layer 314, lower aluminum silicon layer 312,and metal layer 310. The dry oxide etch process stops on the upperaluminum silicon layer 316 on the flat surfaces. This protects thetitanium nitride layer 314. The resulting stack is shown in FIG. 3.

The upper aluminum silicon layer 316 is then removed on the flatsurfaces but not in the via (s) with, for example, a developer such asNMD-W TMAOH in water. The resulting stack is shown in FIG. 4. At thispoint, the top-most layer in the stack is the titanium nitride layer314. The next operation is to pattern the titanium nitride pad out ofthe titanium nitride layer. FIG. 5 illustrates that a pillar pattern 325is placed on the titanium nitride layer 314 at the desired location ofthe titanium nitride pad. The location, as will be seen in FIG. 9,coincides with a portion of the hinge 216 at which the mirror via 202 isto be performed. The cross-sectional shape and size of the pillarpattern 325 matches the desired shape and size of the titanium nitridepad. With the pillar pattern 325 in place, a process operation isperformed to remove titanium nitride everywhere except for the locationof the pillar pattern and in the via(s). Titanium nitride may be removedby, for example, a chlorine and boron trichloride RIE plasma etch. Theetching process etches through the titanium nitride layer 314 and atleast some of the bottom aluminum silicon layer 312. The aluminumsilicon layer 312 ensures that the underlying metal layer 310 is notdisturbed, or that at least an insufficient amount of material from themetal layer 310 is removed to cause any performance issues with thehinge. The pillar pattern 325 is stripped. Then, using a developer suchas NMD-W TMAOH in water, the pillar pattern 325 is stripped and theremaining lower aluminum silicon layer 312 is removed on the flatsurfaces but not from under the titanium nitride pad and not from thevia (s).

FIG. 6 illustrates the resulting stack with the titanium nitride paddepicted as titanium nitride pad 314 a and lower aluminum silicon layerdepicted as lower aluminum silicon pad 312 a. FIG. 7 shows a portion ofthe surface of the metal layer 310. The titanium nitride pad 314 a isshown as are a number of vias, such as address vias 213 and bias vias208. The metal layer 310 is then processed to form the torsion hinge 216thereon. FIG. 8 shows an example in which a mask (not shown) is placedon the metal layer 310 and those portions of the metal layer 310 notcovered by the mask are then etched to thereby create the torsion hinge216. In some embodiments, the etching process to form the hinge 216 maycomprise a chlorine and boron trichloride RIE plasma etch. FIG. 9 showsa portion of the metal layer with a portion of the hinge 216 formedthereon. As can be seen the titanium nitride pad 314 a is located at thecenter of the hinge where the mirror via is to be formed.

FIG. 10 illustrates an alternative embodiment of the materials stack toform the hinge 216 and titanium nitride pad 314 a. This example does notinclude the lower aluminum silicon layer that was otherwise present(aluminum silicon layer 312) in FIGS. 2-9. In the example of FIG. 10, adifferent type of etching process may be used to etch the titaniumnitride layer 314 without removing any or much of the metal layer 310.In one example, the etching process may comprise fluoroform and chlorinein a dry etching process such as a plasma reactive-ion etching (RIE)process, although other types of etching processes may be used as wellSome of the underlying metal layer 310 may be removed as part of thisetching process (e.g., less than approximately 10% of the thickness ofthe metal layer 310) but removal of a relatively small amount of themetal layer does not result in a performance degradation of theresulting hinge. In some embodiments, the metal layer 310 may beinitially deposited to a thicker level than may be needed for the hingeso that some of the metal can be removed when etching the titaniumnitride pad. The layers depicted in FIG. 10 may be sputtered in the sametool in two different process chambers connected under vacuum so that nonative oxide grows between the metal layer 310 and the titanium nitridelayer 314.

Once the hinge 216 is formed, an upper sacrificial layer 330 which maycomprise a photoresist is formed. FIG. 11 illustrates the deposition ofthe upper sacrificial layer 330 on sacrificial layer 308 (also termed“lower” sacrificial layer). The upper sacrificial layer 330 also coversthe titanium nitride pad 314 a. A photolithography process exposes avia(s) 335 over the titanium nitride pad 314 a where a low resistanceinterconnect to the metal mirror layer 340 is to be formed. The mirrormetal 340 (e.g., a reflective aluminum alloy) also deposited on the viawalls and via bottom, as is shown in FIG. 11. Once deposited, the mirrormetal is further processed in a photolithography pattern and plasma RIEetch and cleanup to separate each pixel's mirror from adjacent pixels.In a subsequent packaging process, the lower and upper sacrificiallayers 308 and 330 are selectively removed through the gaps around eachpixel. The metal that formed in the via 335 remains thereby forming themirror via 202 shown in FIG. 1. The mirror via 202 resides on thetitanium nitride pad 314 a which is on the lower aluminum silicon layer312 which is on the hinge 216. FIG. 12 illustrates the embodiment inwhich the lower aluminum silicon layer 312 is absent as noted withrespect to FIG. 10.

FIG. 13 illustrates the titanium nitride pad 314 a formed in a processthat self-aligns the titanium nitride pad to the underlying hinge 216.In this embodiment, the titanium nitride pad is initially oversized(e.g., sized to have a footprint larger than the desired size of thefinal pad). That is, during the etching process to form the titaniumnitride pad 314 a, the pad is formed larger than the area of the mirrorvia to be formed thereon. The excess titanium nitride of the initiallyoversized titanium nitride pad is removed during the subsequent processoperation to form the torsion hinge 216. As a result, the width W1 ofthe hinge 216 and the titanium nitride pad 314 a is the same and thetitanium nitride pad is centered on the hinge.

FIGS. 14-16 illustrate the examples of FIGS. 11-13, respectively, afterthe removal of the sacrificial layers 308 and 330.

In this description, the terms “couple” and “couples” mean either anindirect or direct connection. Thus, if a first structure couples to asecond structure, that connection may be through a direct connection orthrough an indirect connection via other structures and connections.Further, in this description, the term “approximately” means plus orminus 10% in some embodiments.

Modifications are possible in the described embodiments, and otherembodiments are possible, within the scope of the claims.

What is claimed is:
 1. An apparatus, comprising: a semiconductorsubstrate; a hinge having a first side and a second side opposite thefirst side, the first side facing the semiconductor substrate; an etchstop layer on the second side of the hinge; a titanium-based pad on theetch stop layer; a mirror; and a mirror via coupling the mirror and thetitanium-based pad.
 2. The apparatus of claim 1, wherein the hingecomprises metal and the mirror comprises metal.
 3. The apparatus ofclaim 1, further comprising an aluminum silicon layer between thetitanium-based pad and the hinge.
 4. The apparatus of claim 3, whereinthe aluminum silicon layer is approximately 200 {acute over (Å)} thick.5. The apparatus of claim 1, wherein the titanium-based pad has athickness of approximately 50 {acute over (Å)}.
 6. The apparatus ofclaim 1, wherein the titanium-based pad comprises titanium nitride. 7.The apparatus of claim 1, wherein the apparatus is a digital micromirrordevice (DMD).
 8. The apparatus of claim 1, wherein the hinge is atorsion hinge.
 9. The apparatus of claim 1, wherein the hinge isconfigured to rotate the mirror.
 10. The apparatus of claim 1, whereinthe titanium-based pad comprises titanium-tungsten or titanium-aluminumnitride.
 11. A device comprising: a semiconductor substrate; a hingehaving a first side and a second side, the first side of the hingefacing the semiconductor substrate; bias vias coupled to the hinge; anetch stop layer on the second side of the hinge; a titanium-based pad onthe etch stop layer; a mirror; and a mirror via coupling the mirror andthe titanium-based pad.
 12. The device of claim 11, wherein the hingecomprises metal and the mirror comprises metal.
 13. The device of claim11, wherein the titanium-based pad comprises titanium nitride.
 14. Thedevice of claim 11, wherein the titanium-based pad comprisestitanium-tungsten or titanium-aluminum nitride.
 15. The apparatus ofclaim 1, wherein the etch stop layer comprises aluminum silicon.
 16. Theapparatus of claim 1, wherein the titanium-based pad is shaped on thehinge before the hinge is shaped.
 17. The device of claim 11, whereinthe etch stop layer comprises aluminum silicon.
 18. The device of claim11, wherein the titanium-based pad is shaped on the hinge before thehinge is shaped.
 19. The device of claim 11, wherein the etch stoplayer, the titanium-base pad, and the mirror via are configured toconduct a bias voltage received through the bias vias to the mirror.